Electronic dance music (EDM) nightclub and festival attendees are at high risk for drug use and associated adverse outcomes. EDM attendees are at high risk for using ?traditional? drugs such as ecstasy (MDMA, ?Molly?), but they are also now at high risk for use of new psychoactive substances (NPS), which are a major new drug-related public health threat. NPS are often adulterants in or replacements for ?traditional? illicit drugs such as ecstasy; thus, many users unknowingly ingest such new and dangerous substances. Ecstasy, for example, is often adulterated or replaced with NPS such as synthetic cathinones (?bath salts?) and the PI and his colleagues have detected highly potent drugs (e.g., alpha-PVP, PMMA, methamphetamine) in ecstasy users' hair despite them denying use of these drugs. Thus, ecstasy use appears to be a major delivery system of unintentional NPS use. Fentanyl and other dangerous opioid NPS are also now being detected in cocaine and in pills resembling Xanax and Percocet. Drug-related deaths at EDM festivals have become common, and with the popularity of EDM parties increasing, it is essential to focus on intentional and unintentional drug use in this at-risk population in order to prevent use, poisonings, and deaths. We have developed and validated a rapid survey (via the PI's K01) to query use of dozens of common drugs and >100 NPS in the EDM scene and we have validated hair testing methods to determine use of >100 NPS and >15 common drugs. Such biological testing can help us estimate the true prevalence of use of NPS and other drugs (e.g., ecstasy) in these high- risk nightlife scenes. The objective of this study is to systematically examine the prevalence of known and unknown drug use, risk factors for use, self-reported adverse outcomes associated with use, and trends in use and adverse outcomes over 24 months. Since many individuals unknowingly use NPS, it is essential to validate survey responses. This study is innovative as we will test hair of participants to validate responses and also determine the extent of unintentional use of NPS and other drugs. Hair testing is an ideal biological testing method because we are able to detect the presence of >100 NPS (including fentanyl and 15 of its analogues) in hair for >12 months post-use. Our validated rapid survey complemented with hair testing (for validation) will allow us to estimate the prevalence of drug use in the nightclub and festival scenes in NYC. Using time-space sampling, we will randomly select EDM parties throughout NYC, and survey 2,080 individuals entering these parties (biweekly). An exploratory longitudinal component will also help us determine whether providing a subsample of participants with feedback regarding their hair testing results influences drug use and intention to use over time (6 months post-survey). Estimating intentional and unintentional drug use and adverse consequences associated with use may allow us to detect some risky drug trends (e.g., use of [adulterated] ecstasy) before diffusing to the general public. Methods will be generalizable to other at-risk populations and results will inform prevention information to be disseminated to the public and to those at highest risk.